Amateur Radio (G3TXQ) - G5RV Antenna

Few antennas polarise opinion as much as the G5RV - it seems you either swear by it or swear at
it! The purpose of this article is to present a few technical facts about the antenna and allow
the reader to judge whether or not it might be useful for them.

A. What is a G5RV?

Ask this question on one of the Internet Ham Radio discussion forums and you will almost
certainly get a diversity of opinion. The problem partly lies with Louis Varney (G5RV) himself who
variously described the antenna as having: a matching section of open-wire line, or 300 ohm line;
a feeder comprising 50 ohm coax, 80 ohm coax, 75 0hm twin-lead, or no feeder at all; a balun, no
balun, or an HF choke!

For the purposes of this discussion we shall assume that the antenna comprises:

A centre-fed dipole which is three half-waves long on 20m - shown red (A) in the diagram

A ‘matching section’ of 300 ohm ladderline an electrical half-wave long on 20m -
shown blue (B) in the diagram

A length of 50 ohm coax of unspecific length - shown green (C) in the diagram

The need for a balun at the ladderline-to-coax junction - point X - is discussed later

B. What bands does it cover?

Again you will find a wide range of opinions. There are those who claim that Varney only ever intended it
to be a monoband 20m antenna, and those (often commercial antenna suppliers) who claim that it is
an ‘all-band’ 80m thru 10m antenna. Certainly Varney discussed its performance on all
bands 160m thru 10m (including the WARC bands), so it seems likely he thought of it as something
other than a monoband antenna.

Band

Best VSWR

Worst VSWR

160m

>100

>100

80m

3.2

12.6

40m

4.9

5.9

30m

48

49.5

20m

2.5

3.7

17m

32.1

33.6

15m

6.1

12.9

12m

3.6

4.6

10m

51

59.6

Let's model a G5RV in EZNEC and look at the feedpoint impedances. We will assume the dipole
is built with #14 bare copper wire at a height of 30ft over average ground, and that the matching
section is 300 ohm ladderline with a velocity factor of 0.9 and a loss of 0.2dB/100ft at 30 MHz.
In building the model we firstly adjust the length of the dipole to be resonant at 14,150kHz, and
then adjust the length of the matching section to be an exact half-wave at this frequency; we find
the dipole needs to be 103.25ft long and the matching section 31.2 ft long (34.7ft * Vf).

The table on the right lists the worst and best 50 ohm VSWRs presented at point X for each amateur band.
These are close to the VSWRs that would be seen by a transceiver if a very short length of low-loss
coax were used. We notice that:

On no part of any band does the VSWR fall below 2:1 - a typical limit for a modern
solid-state transceiver

All of 20m, and parts of 80m and 12m, have a VSWR below 4:1 - a typical limit for a
modern transceiver with built-in auto tuner

80m, 40m, 20m, 15m, and 12m exhibit moderate VSWRs which should be within the matching
range of an external tuner, and for which the losses in a short length of good-quality
coax would likely be acceptable

Band

Best VSWR

Coax loss

160m

20.7

20dB

80m

2.8

0.8dB

40m

3.3

1.9dB

30m

8.2

7.7dB

20m

2.0

1.5dB

17m

5.5

7.8dB

15m

3.3

3.1dB

12m

2.4

2.4dB

10m

4.8

10.5dB

In an attempt to improve these VSWR figures, some commercial vendors supply the antenna with
a long length of relatively lossy coax feeder, claiming (rightly) that this length is important
to the working of the antenna. Let's run our model again with 70ft of RG58 coax between the
ladderline and the rig. The table on the right shows the new VSWRs and the losses in the coax.
We see that:

20m is now just about usable without any sort of tuner, at a cost of 1.5dB loss

80m, 40m, 20m, 15m and 12m would likely be within range of a built-in auto-tuner but we
would have to accept losses over 3dB (for 15m)

All bands 80m thru 10m would be within the matching range of a good external tuner, but the
losses on 30m, 17m, and 10m would produce mediocre performance

160m is unusable [Varney only ever suggested using the antenna on 160m as a top-loaded
vertical with the ladderline wires strapped together and fed against ground]

Band

VSWR range(EZNEC)

VSWR range(Measured)

80m

10 - 3.1

8.8 - 3.1

40m

5.7 - 4.9

6.2 - 5.6

30m

18.3 - 18.5

>10

20m

4.4 - 2.6

4 - 2.3

17m

13.4 - 13.2

>10

15m

3.2 - 6.3

2.8 - 4

12m

1.8 - 2.1

2.0 - 2.0

10m

14

>10

Finally, this table compares VSWR measurements made on a real G5RV with EZNEC predictions.
The antenna was mounted in an Inverted-V configuration with the apex at just 18ft; the ends were
at a height of 6ft. A 1:1 current balun was in place at the base of the ladderline, and the coax
section comprised approximately 18ft of RG213.

There is generally good agreement between the predicted and measured results - certainly close
enough for most amateur requirements. The VSWRs are a little different from those in the earlier
tables because of the low Inverted-V configuration. Despite this significantly different
configuration, the conclusions about band coverage are no different.

We conclude that, from a matching perspective, the G5RV is far from being an all-band antenna.
With low losses in the coax section, the antenna cannot be used on any band without some kind of
tuner. To quote Varney: "The use of an unbalanced-to-unbalanced matching network between the coaxial
output of a modern transmitter (or transceiver) and the coaxial feeder is essential. This is because
of the reactive condition presented at the station end of this feeder, which on all but the
14-MHz band, will have a fairly high to high SWR on it". With a modern in-built auto-tuner, it's
at best a 3-band antenna (80m, 20m, 12m). With a good external tuner it's probably a 5-band
antenna (80m, 40m, 20m, 15m, 12m). Other bands can be brought within matching range by incurring
losses in the coax!

Of course by using a good external tuner and avoiding the coax section by bringing the
ladderline all the way to the shack, losses will likely be acceptable on all bands 80m thru 10m
......... but is the antenna then a G5RV or simply a 102ft multiband doublet? Given that it was
one of the feed systems described by Varney in his original 1958 article, perhaps it still is
a G5RV!

Like any multiband doublet the G5RV exhibits a different azimuth response on the various
bands. By way of example, the diagram on the right shows the Free Space 80m response (blue), the
20m response (red) and the 15m response (green). On the lower frequencies the azimuth response is two
lobes, each broadside to the antenna. As the frequency increases the response becomes more complex
with multiple lobes and nulls. These features become less pronounced when the antenna is modelled
above real ground, but nonetheless nulls up to 20dB deep are possible and it is worth taking account
of them when planning the orientation of the antenna.

C. Balun or no balun?

It is good engineering practise to fit a 1:1 current balun at any coax/balanced transition in
an antenna system; it helps prevent common-mode currents on the outside surface of the coax braid
which may cause "RF in the shack" problems when transmitting and local noise pick-up when
receiving. Varney advocated a balun in his original article, but in his 1984 Radio Communication
article he changed his mind because he felt that the reactive loads could result in heating of
the windings and saturation of its core. Even then he advocated a "coaxial cable HF choke".

With modern ferrite materials, and our better understanding of balun characteristics than in
Varney's day, there is no reason not to include a ferrite-cored 1:1 Guanella balun at the
ladderline/coax interface.

The diagram on the right shows what can happen if you don't include the balun; it is an EZNEC
model of a G5RV antenna system. Wires 1 & 2 represent the antenna, wires 3 & 4 the vertical
ladderline section, and wire 6 the coax section. The purple lines show the current distribution
along the wires.

Notice that:

The currents in the two dipole legs are unequal; this will cause "skewing" of the azimuth
patterns

The currents in the two ladderline legs are unequal; this will cause radiation from the
vertical section on Tx

There is considerable common-mode current flowing on the coax - coax which ends up close
to the radio and other household equipment; this can cause RFI problems on Tx, and on Rx noise
from that equipment can be injected at the coax/ladderline junction

Now see how the situation improves when we include a 1:1 balun (or common-mode choke) - the
currents in the two dipole legs and the two ladderline legs are well-balanced, and there is
negligible common-mode current on the coax.

D. Can we do better?

Band

Best VSWR

Worst VSWR

160m

>100

>100

80m

8.3

18.8

40m

1.1

1.4

30m

87

89

20m

1.2

3.2

17m

1.4

1.6

15m

80

90

12m

1.2

1.4

10m

1.5

9.7

Brian Austin ZS6BKW/G0GSF searched for combinations of antenna length and feedline length
which would provide a reasonable match to 50 ohms on a number of HF bands; he was succesful - he
found that an antenna 93ft long fed with 39.8ft of 400 ohm ladderline (Vf=0.9) provides a
reasonable match on five bands. The table on the right shows the best and worst VSWRs of this
antenna system on each HF band modelled at a height of 30ft above average ground.

The antenna would be usable on all of 40m, 17m and 12m without any tuner, and much of 20m and
10m. Perhaps its greatest disadvantage is that it does not cover the popular 80m and 15m
bands without a tuner.

When LB Cebik analysed the G5RV and variants, he concluded: "Of all the G5RV antenna system
cousins, the ZS6BKW/G0GSF antenna system has come closest to achieving the goal that is part of
the G5RV mythology: a multi-band HF antenna consisting of a single wire and simple matching system
to cover as many of the amateur HF bands as possible. From 80 to 10 meters, Austin's system
provides an acceptable match on 5 out of the 8 bands under most conditions without an antenna
tuner. This is the best result that has been achieved of any of the systems that has come to my
attention."